Basic Cryptography

Question 1

What is cryptography?

Answer 1

refers to the art and science of concealing meaning

Question 2

What is a cryptosystem?

Answer 2

a system where a sender/transmitter transforms all unconcealed data (plaintext) into concealed data called ciphertext using an encryption algorithm
The receiver then transforms the received ciphertext back to the plaintext using a decryption algorithm

Question 3

What is a cipher?

Answer 3

encryption and decryption algorithms

Question 4

What are the two broad types of cryptosystems?

Answer 4

Private key and public key

Question 5

What is the encrypt function

Answer 5

EM = E(K, M)

Question 6

What is the decrypt algorithm

Answer 6

M = D(K, EM)

Question 7

What is “Simple Division by Key”

Answer 7

The secret key is a number, each letter is assigned a numbered value and then encoded by dividing that number by the key. The letter is then the quotient and the remainder (2 digits)

Question 8

What are the two types of historical cryptosystems?

Answer 8

Substitution ciphers (value of plaintext is replaced), and Transposition ciphers (position of plaintext alphabet are changed without changing their values)

Question 9

What are the two types of substitution ciphers?

Answer 9

Monoalphabetic and Polyalphabetic

Question 10

What is the beale cipher

Answer 10

a polyalphabetic substitution cipher.

The key is a string of letters, the plaintext is written under the key, each plaintext character is shifted by value

Question 11

What is the vigenere cipher

Answer 11

The key is represented by a string of letters. Each key letter represents a certain shift of the plaintext character.
Find the PT letter row and the Key letter column to find the CT character

Question 12

What is the digraphic substitution cipher

Answer 12

Create a 5x5 table. Begin to fill the table with the key, then the rest of the alphabet excluding the letters that exist in the key.
Divide the PT into groups of two characters.
Find a rectangle that contains each group, switch the corner characters to get the CT.
If there are only two characters in the same row, wrap the row left to right. If there are only two characters in the same column wrap to the next character in the column

Question 13

What is a rail fence cipher?

Answer 13

write the plaintext in 2 rows. Then write the cipher text out by writing the first row and then the second

Question 14

What is the matrix cipher?

Answer 14

Write the plaintext in a matrix of n columns rowwise.

Create a key and use it to determine the order and write the CT by taking the columns in the order dictated by the key.

Question 15

What was the Enigma Machine?

Answer 15

A polyalphabetic substitution cipher. It used a machine with mechanical rotors that changed the electronic circuitry for ciphering.

Question 16

What is a one-time pad?

Answer 16

A one time pad consists of a set of non-repetitive keys where each key consists of a truly random set of letters. The keys are written on separate sheets of paper, and the sheets are glued to a pad.

Each time a message is transmitted a new key is selected from the top of the pad. The receiver has an identical set of keys, once the key has been used, both sender and received discard that key.

In current systems, this concept is used in pseudo-random number generators for stream ciphers.

Question 17

Why aren’t traditional private key cryptosystems robust?

Answer 17

• The key can be found after reverse engineering using known plaintext attacks
• The fact that certain letters of the English alphabet are more frequent than others allows for educated guesses to be made
• Traditional systems are not robust against cryptanalysis

Question 18

What can be done to improve traditional private key cryptosystems?

Answer 18

• Better ciphers must prevent the attacker from reverse engineering the key
• Make the encryption algorithm so complicated that none of the plaintext remains in the corresponding ciphertext
• Two categories of such ciphers are Block ciphers and Stream ciphers

Question 19

Describe the structure of a Block Cipher diagram

Answer 19

Plaintext (in blocks of size m bits) undergoes a block cipher algorithm with a key of substitution, permutation, and looping to output Ciphertext (in blockes of m-bits size)`

Question 20

What is DES?

Answer 20

• DES stands for data encryption standard

- has an initial permutation, 16 rounds of processing, a final permutation on each 64-bit input block

Question 21

Describe the overall DES structure diagram

Answer 21

• a 64-bit input block undergoes an initial permutation.
• a 56-bit secret key gets broken into 16 - 48-bit keys
• The input block undergoes 16 rounds of processing using each of the 16 generated keys
• A final permutation is performed giving the 64-bit ciphertext output block.

Question 22

How does DES achieve security?

Answer 22

It achieves security by confusion and diffusion

Question 23

What is the drawback of ECB (electronic code book)

Answer 23

Identical plaintext blocks give identical ciphertext blocks. This can help in reverse engineering the key.

Question 24

What are the drawbacks of Cipher Block Chaining (CBC)?

Answer 24

• If there is an error in one block, it will propagate to all subsequent blockes
• Sequential processing is slow

Question 25

Why does 3-DES use EDE and not EEE (where E is encrypt and D is decrypt)

Answer 25

EEE would cancel out one E which would reduce brute force attack time . With EDE there is no cancellation.

Question 26

What is a stream cipher?

Answer 26

A stream cipher generates a continuous bit stream from a given plaintext bit stream by encrypting the plaintext on a bit by bit basis

Question 27

How do stream ciphers generate a key stream from a secret keys?

Answer 27

All stream ciphers use pseudorandom sequence generators (PSG) to generate a key stream from a secret key

• A pseudorandom sequence is initialized by the secret key
• Not mathematically random but passes most tests for randomness
• plaintext is XORed with the sequence bit by bit to generate the ciphertext
• The receiver uses the same secret key as a seed to generate the pseudorandom sequence and XORs it with the ciphertext stream bit by bit to produce the plaintext

Question 28

What are the advantages of a stream cipher?

Answer 28

• High throughput
• light weight
• bit errors are not propagated

Question 29

What are the disadvantages of a stream cipher?

Answer 29

Sender and receiver synchronization is required

Strength of the cipher relies on the strength of the PSG